The present invention relates to a device to improve wastewater treatment and particularly the conversion of dissolved solids to suspended solids so they may be removed from the wastewater.
Generally each house or business has a pipe or sewer which connects to a network of larger sewers carrying wastewater to a wastewater treatment plant. The wastewater from a sewer system either flows by gravity or is pumped into the treatment plant. Usually, treatment consists of two major steps, primary and secondary, along with a process to dispose of solids removed during the two steps.
In primary treatment, the objective is to physically remove suspended solids from the wastewater either by screening, settling or floating. Screening removes large floating objects from the incoming wastewater stream. Treatment plant screens are sturdily built to withstand the flow of untreated wastewater for years at a time. Rags, wood, plastics and other floating objects can clog pipes and disable treatment plant pumps if not removed at this point. Sand, grit and gravel flow through the screens to be picked up in the next stage of primary treatment--the grit chamber. Grit chambers are large tanks designed to slow the wastewater down just long enough for the grit to drop to the bottom. Grit is usually washed after its removal from the chamber and buried in a landfill.
After the flow passes out of the grit chamber, it enters a more sophisticated settling basin called a sedimentation tank. Sedimentation removes the solids that are too light to fall out in the grit chamber. The sedimentation tanks are designed to hold wastewater for several hours. During that time, the suspended solids drift to the bottom of the tank where they can be pushed into a large mass by mechanical scraper and removed from the bottom of the tank. The solids removed at this point are called primary sludge. The primary sludge is usually pumped through a sludge digester for further treatment.
During the sedimentation process, floatable substances, such as grease and oil, rise to the surface and are removed by a surface skimming system. The skimmed materials are either sent to the sludge digester for treatment along with the primary sludge, or are incinerated. Sedimentation marks the end of primary treatment. At this point, most of the solids in the stream that can be removed by the purely physical processes of screening, skimming and settling have been collected. An additional set of techniques using biological processes are employed in secondary treatment.
The major goal of secondary treatment is to biologically remove contaminants that are dissolved in the wastewater. In a natural stream such contaminants are a source of food for protozoa, fungi, algae and hundreds of varieties of bacteria. The secondary treatment stage is a highly controlled artificial environment in which the same microscopic organisms are allowed to work as fast and as efficiently as they can. Air is supplied to encourage the natural growth processes of bacteria and other biological organisms to consume most of the waste. The microorganisms biologically convert the dissolved solids in the wastewater to suspended solids which will physically settle out at the end of secondary treatment. These organisms, and other solids, are then separated from the wastewater.
Secondary treatment promotes the growth of millions of microorganisms, bringing them into close contact with the wastewater in which they feed. It is critical to make sure that the temperature, oxygen level, and contact time support rapid and complete consumption of the dissolved wastes. The final products are carbon dioxide, water, and more organisms. Three common types of secondary treatment include trickling filters, activated sludge, and lagoons.
Trickling filters are large beds of coarse, loosely packed material, such as rocks, wooded slats, or shaped plastic pieces, over which the wastewater is sprayed or spread. The surfaces of the filter material ("medium") become breeding grounds for the microorganisms that consume the wastes. A common trickling filter is a bed of stones three to ten feet deep. Under the bed a system of drains collect the treated wastewater and divert it to a sedimentation tank, or back over the filter medium for additional cleansing. In the sedimentation tank, suspended solids settle and are pumped to the sludge digester. Trickling filters are relatively simple to construct and operate.
The dimensions of a trickling filter depend largely upon capacity. In general, the medium of the trickling filter is constrained by a cylindrical concrete or metallic vessel which may or may not be recessed within the earth. Concentric with the vessel is a rotating distribution head having a plurality of distribution arms radiating therefrom, each substantially parallel to the upper surface of the filter medium. Each arm has a plurality of outlet openings or jets adapted to distribute the wastewater over the filter medium. The rotary action of the distributor is induced by reaction from the jets oriented in the same direction. The rate of rotation of the distributor generally increases with an increase in the amount of wastewater distributed. A variable rotation rate of the distributor results in periods of increased build-up of bacterial growth along the upper portion of the filter medium, impeding the filtering process. It is believed that the increased build-up of bacterial growth in the top portion of the filter occurs at the expense of bacterial breakdown further down, thus resulting in an overall decrease in efficiency.
In one instance the rotation rate of wastewater trickling filters has been controlled by the angular orientation of the jets. To increase rotation rate, the distribution arms are rotated about the axis of each arm so that all the jets are oriented in the same direction. The distributor may be slowed by orienting some outlets in the opposite direction or angled slightly with respect to the rest. Passive techniques include air brakes or other wind resistance devices to slow rotation. Active mechanisms include the use of belt or chain drives interconnecting a sheave or gear on the central distribution head to a fixed brake or electric dynamo.
In other attempts, positive drive mechanisms have been placed at the ends of the distribution arms to rotate the trickling filter. Previous devices include electrically driven carriages riding on a monorail located about the perimeter of the filter vessel. Earlier drive mechanisms were powered by gears engaging the perimeter of the vessel and driven by paddlewheels located in the wastewater line.
The disadvantage associated with water propelled tricking filters is that rotation rate increases with increased flow. The net affect is that the flow rate per unit area of medium is not substantially changed, resulting in foaming and increased bacterial growth in the uppermost reaches of the medium. Efficiency of the filter is not improved. Positive drive mechanisms require a power source independent of wastewater flow such as electricity. This results in more complex and expensive mechanisms to operate.
The present invention was devised as a solution for these and other problems by providing a device to control the maximum rotation rate of the distribution arms thereby improving the hydraulic action of the wastewater over the filter medium to promote bacterial growth and removal of dissolved solids. Moreover, the present invention is not affected by wastewater discharge and provides an inexpensive and reliable independent control.